This invention relates to a surface acoustic wave device.
A surface elastic wave is ordinarily called a surface acoustic wave (SAW) in the art. A surface acoustic wave device comprises a substrate of a piezoelectric material, such as quartz, having a principal surface and at least two electrode members on the principal surface. Each electrode member serves as a transducer between an electric signal and surface acoustic waves which are propagated along the principal surface. The surface acoustic wave device is used in electric communication apparatus usually as a band-pass filter. Consequently, the surface acoustic wave device must have an excellent outband characteristic in addition to an excellent passband characteristic. Furthermore, the surface acoustic wave device must have a small insertion loss.
Various surface acoustic wave devices are already known. For example, a conventional surface acoustic wave device comprises first through third interdigital electrode members. It is known in the art that an interdigital electrode member comprises a pair of parallel strip electrodes and a plurality of interdigital electrodes extended perpendicularly from the strip electrodes in an interdigital fashion with a predetermined width of interlock formed along each pair of interdigital electrodes extended from the respective strip electrodes. For convenience of the description which follows, each interdigital electrode member will be said to have first and second ends at two of the interdigital electrodes that are extended at outermost extremities.
The first through the third interdigital electrode members are arranged on the principal surface of the substrate in succession with the strip electrodes disposed colinear and with a space left between the first and the second ends of two adjacent interdigital electrode members. The surface acoustic wave device is used as a three-transducer device. More particularly, the second interdigital electrode member is used as a driving transducer with the first and the third interdigital electrode members used as driven transducers. Alternatively, the first and the third interdigital electrode members are used as driving transducers with the second interdigital electrode member used as a driven transducer.
When supplied with an input electric signal of a high frequency, the driving transducer excites surface acoustic waves which travel along the principal surface perpendicularly of the interdigital electrodes with the high frequency and with a wavelength dependent on the substrate. Each pair of the interdigital electrodes has a center-to-center spacing which is equal to a half of the wavelength. The driven transducer transduces the surface acoustic wave arriving thereat into an output electric signal.
Another conventional surface acoustic wave device comprises two apodized electrode members as a driving and a driven transducer. In the manner which will later be described in detail, an apodized electrode member comprises a pair of parallel strip electrodes and a plurality of finger electrodes which are perpendicularly extended from the strip electrodes and have weighted finger lengths perpendicularly of the strip electrodes. Each pair of the finger electrodes has a center-to-center spacing which is equal to a half of the wavelength of the surface acoustic waves. Like the interdigital electrode member, each apodized electrode member has first and second extremities at two of the finger electrodes that are most outwardly arranged. Besides an apodized electrode member having a substantially linear-phase impulse response, a different apodized electrode member is known which has substantially minimum-phase and maximum-phase impulse responses for the surface acoustic waves propagated through one and the other of the first and the second extremities, respectively.
Still another conventional surface acoustic wave device comprises an interdigital electrode member and an apodized electrode member. When either no or only one apodized electrode member is included, the surface acoustic wave device has not sufficiently excellent outband characteristics. When only two electrode members are used, the surface acoustic wave device has a considerably great insertion loss.